1. Field of the Invention
The present invention relates to a method for manufacturing a capacitor using a ferroelectric thin layer, and more particularly to a DC magnetron reactive sputtering method for manufacturing an electrode of such a capacitor.
2. Description of the Related Art
In a dynamic random access memory (DRAM) device, the integration has become highly developed. Generally, one memory cell of a DRAM device is constructed by one transistor and one capacitor. Therefore, in order to enhance the integration, as the transistor needs to be fined, the capacitor also needs to be fined, which reduces the storage capacitance of the capacitor.
On the other hand, in order to suppress the above-mentioned reduction of the storage capacitance of the capacitor, a ferroelectric layer made of ferroelectric material such as Pb (Zr, Ti) O.sub.3 (PZT) or (Ba, Sr) TiO.sub.3 (BST) has been used as a high-dielectric layer. Note that, if such a ferroelectric layer is used in a capacitor of a DRAM device, even after the power supply is turned OFF, data stored in the capacitor is not erased. Therefore, the DRAM device can serve as a nonvolatile memory which is called a ferroelectric RAM (FeRAM) device.
Since the ferroelectric material is a kind of compound metal oxide, electrodes used for a ferroelectric capacitor are preferably made of material which does not allow the ferroelectric material to be reduced (deoxidized). Note that, if the ferroelectric material is reduced, the leakage current is increased, the dielectric constant is decreased, and the remanent polarization is decreased, thus deteriorating the capacitor characteristics.
As the above-mentioned electrode materials, noble metal such as platinum (Pt) and conductive oxide such as iridium oxide (IrO.sub.2) are known. Particularly, in view of good polarization fatigue properties, IrO.sub.2 is promising as the electrode material, since IrO.sub.2 exhibits excellent barrier performance against oxygen (see JP-A-7-245237).
In a first prior art method for manufacturing an IrO.sub.2 electrode for PZT as ferroelectric material (see T. Nakamura et al., "Preparation of Pb (Zr, Ti) O.sub.3 thin films on electrodes including IrO.sub.2 ", Appl. Phys. lett. 65 (12), pp. 1522-1524, September 1994), an IrO.sub.2 lower electrode and an IrO.sub.2 upper electrode are deposited on a substrate by an RF magnetron reactive sputtering process using an Ir target and Ar/O.sub.2 mixture gas without heating the substrate, i.e., at room temperature. Then, the IrO.sub.2 electrodes are annealed at 400.degree. C. for 15 minutes. Thus, the polarization fatigue characteristics of PZT can be improved.
In the above-described first prior art method, however, even after the annealing process is completed, Ir is not combined sufficiently with oxygen, so that an Ir phase exists in an interface with PZT. Therefore, the Ir phase may slightly interact with PZT at their interface, which deteriorates the polarization fatigue characteristics (see Hag-Ju Cho et al., "Preparation and characterization of Iridium Oxide Thin Films Deposited by DC Reactive Sputtering", Jpn. J. Appl. Phys. Vol. 36, part 1, No. 3B, pp. 1722-1727, March 1997).
Note that since IrO.sub.2 serves as an excellent barrier against oxygen, IrO.sub.2 shuts off the supply of oxygen to PZT while an Ir phase generated by the incomplete oxidation of Ir absorbs the oxygen of PZT. Therefore, if IrO.sub.2 is used as the upper electrode of PZT, the above-mentioned deterioration of the polarization fatigue characteristics may occur. On the other hand, if IrO.sub.2 is used as the lower electrode of PZT, the interface between the IrO.sub.2 lower electrode and PZT may be short of oxygen; however, this shortage of oxygen can be compensated for by the supply of oxygen from the upper surface of PZT.
In a second prior art method for manufacturing an IrO.sub.2 electrode for PZT as ferroelectric material (see M. Shimizu et al., "Pb (Zr, Ti) O.sub.3 Thin Film Deposition on Ir and IrO.sub.2 Electrodes by MOCVD", Journal of the Korean Physical Society, Vol. 32, pp. S1349-S1352, February 1998), an IrO.sub.2 electrode for PZT is deposited on a substrate by an RF magnetron reactive sputtering process using an Ir target and Ar/O.sub.2 mixture gas while heating the substrate at a high temperature such as 530.degree. C. Thus, the IrO.sub.2 electrode is completely oxidized, which dissolves the Ir phase at the interface between the IrO.sub.2 electrode and PZT.
However, if the above-described second prior art method is applied to the upper electrode of PZT, the upper surface of PZT is exposed to a high temperature under a low pressure, thus changing the properties of the upper surface of PZT. Therefore, the polarization fatigue characteristics of PZT is deteriorated.
In view of the foregoing, in a method for manufacturing conductive oxide electrodes for a ferroelectric capacitor, the interface between the conductive oxide electrodes and the ferroelectric material should be sufficiently oxidized at a relatively low temperature which avoids the deterioration of the properties of the surfaces of the ferroelectric material.
Additionally, generally, each conductive oxide electrode for a ferroelectric capacitor needs to be connected to an interconnect (wiring) layer. In this case, if the conductive oxide electrode is connected directly to the wiring layer, the material of the wiring layer may be oxidized by oxygen included in the conductive oxide electrode, which invites disinterconnect therebetween. Therefore, each electrode for the ferroelectric capacitor is preferably constructed by a conductive oxide layer and a metal layer which has a good adhesion to the material of the wiring layer. For example, the electrode for PZT is made of Ir/IrO.sub.2 (see JP-A -8-51165 & JP-A-9-148535). However, no concrete method for manufacturing such Ir/IrO.sub.2 electrodes has been known.